The information provided below is not admitted to be prior art to the present invention, but is provided solely to assist the understanding of the reader.
Screen printing is considered to be one of the major printing processes due to the versatile nature of the process, economy of short run work and the relatively low capital investment required to start a screen printing process. Solvent-based evaporation drying ink systems are still the most widely used technology in screen printing, but increased emphasis on environmental considerations has been a driving force in advancing the utilization of UV technology in screen printing. Advances in raw material technology have made it possible to formulate screen inks which can UV cure at commercial line speeds to give prints with good opacity, durability and chemical resistance. It is important to use a substantial amount of a mixture of different photo initiators to ensure surface and through cure in a thick film application such as screen printing. Traditional photo-initiators (e.g., benzophenone) can be toxic, expensive, and malodorous. An additional drawback to traditional photo-initiators is their tendency to contribute to film color, which can limit their applicability to white and light-colored inks.
In recent years, UV flexographic inks have developed to become a serious alternative to offset or letterpress printing inks as well as to solvent and water-based flexographic inks. Environmental concerns mitigating the use of volatile solvents, financial considerations, and the availability of alternative technologies, have combined to persuade printers to consider UV flexographic printing as a viable low cost option. Advances in raw material technology have made it possible to formulate flexo inks which have the high pigment loadings and good flow characteristics that are needed in the ink fountain, to enable the ink to transfer cleanly from the anilox roll, as well as to print with high color density from fine line aniloxes. In addition, better flow properties lead to a better printed appearance, as the ink will level more completely and yield good gloss. The flexographic inks of the present invention can UV-cure at commercial line speeds to give prints with good opacity, durability, and chemical resistance. Current commercial UV-cure flexo inks require the use of substantial quantities of a mixture of different photo initiators to ensure optimum cure and to obtain proper adhesion in thin film applications under the various conditions applicable to flexographic printing. Traditional photo initiators (e.g., benzophenone) are undesirable because they can be toxic, expensive, and malodorous. Moreover, their use can contribute to film color, which can limit applicability in white and light-colored inks.
Acrylate, methacrylate and other unsaturated monomers are widely used in coatings, adhesives, sealants, and elastomers, and may be crosslinked by ultraviolet light in the presence of photoinitiators or by peroxide-initiated free radical cure. These photoinitiators and/or peroxides are typically low molecular weight multifunctional compounds that may be volatile or readily absorbed through skin and can cause adverse health effects. Functionalized oligomeric photoinitiators may overcome some of these drawbacks; generally, polymeric photoinitiators are nonvolatile compounds, not readily absorbed through skin. However, multistep syntheses may be required, low functionality may be detrimental to reactivity and final properties, and catalyst or initiator may still be required to affect crosslinking.
The Michael addition of acetoacetate donor compounds to multiacrylate receptor compounds to make crosslinked polymers has been described in the literature. For example, Mozner and Rheinberger reported the Michael addition of acetoacetates to triacrylates and tetracrylates. (16 Macromolecular Rapid Communications 135 (1995)). The products formed were crosslinked gels. In one such reaction, depicted in FIG. 1, Mozner added one mole of trimethylol propane triacrylate (TMPTA) having 3 functional groups to one mole of polyethylene glycol (600 molecular weight) diacetoacetate (PEG600-DAA) having two functional groups. (Each acetoacetate “functional group” reacts twice, thus each mole of diacetoacetate has four reactive equivalents.) The resulting network is considered “gelled”, or cured, despite the presence of unreacted acrylic functional groups. While further reaction can be promoted, this network cannot be made liquid either with heat or solvent because it is essentially crosslinked.
U.S. Pat. Nos. 5,945,489 and 6,025,410, to Moy et al. and assigned to the assignee of the present invention, disclose that certain soluble liquid uncrosslinked oligomers, made by one step Michael addition of β-dicarbonyl donor compounds (e.g., acetoacetates) to multifunctional acrylates, can be further crosslinked using ultraviolet light without requiring costly photoinitiators. Moreover, when precise proportions of multiacrylate acceptor compounds to β-dicarbonyl donor compounds are combined in the presence of a basic catalyst, liquid oligomeric compositions result. If proportions below the ranges disclosed in the above-cited patent documents are used, crosslinked gels or solid products are made. In addition, the disclosed liquid oligomer compositions can readily be applied to various substrates using conventional coating techniques such as roll or spray prior to ultraviolet light cure.
The amount of external photoinitiator in ink formulations can be significantly reduced by using the acrylate oligomer technology described in patents U.S. Pat. No. 5,945,489 and U.S. Pat. No. 6,025,410 (both Ashland, Inc.). The invention disclosed here demonstrates the advantageous use of these uncrosslinked resins alone or modified by reaction/blending with additional materials in ink formulations for flexo and screen-printing applications. These additional materials include a variety of acrylic monomers and oligomers; vinyl monomers, such as N-vinyl caprolactam and vinyl ethers; primary, secondary, and tertiary amines; acid-functional materials; siloxanes; elastomers; waxes; and other materials to modify and improve printing performance and pigments to impart color to inks.
Screen-printing ink formulations comprised of traditional monomers and oligomers often have compatibility issues with some of the above additives. Therefore, the designer of traditional screen-printing inks has fewer formulation options. However, formulations built from the novel photo-curable oligomer resins described herein can incorporate a nearly unlimited variety of additives due to the chemical/architectural control possible in their synthesis. Thus, many more options are available to the formulator who must address specific challenges (e.g., adhesion, flexibility, etc.) for each particular substrate.
Flexo and Screen-printing inks based on the resins described above can be cured by all methods typically used to crosslink acrylic materials. Cure, or crosslinking, is usually accomplished through a free radical chain mechanism, which may require any of a number of free radical-generating species such as peroxides, hydroperoxides, REDOX combinations, and other materials that decompose to form radicals either when heated, or at ambient temperature in the presence of a promoter such as an amine or a transition metal. Free radicals are also generated by the decomposition of a photo-initiator under the influence of ultraviolet (UV) or electron beam (EB) radiation.
Flexo and Screen-printing inks based on the novel acrylate oligomers described in this invention offer significant advantages over inks based on traditional multifunctional acrylic monomers and oligomers in that they can be cured by exposure to UV radiation with a fraction of the photo-initiator required for standard UV-cure inks. Under typical UV curing conditions (˜300−600 mJ/cm2), these inks can be effectively cured on a variety of substrates with substantially less photo-initiator.
Other objects and advantages will become apparent from the following disclosure.